Power line disturbances can produce serious problems for continuous process industries. In particular, voltage sags and dropouts can be some of the most troublesome events. Events lasting less than one-half second can shut down a facility and halt production for several hours, which can result in a substantial cost to the manufacturer due to loss of product. Industrial equipment reacts to voltage sags in a variety of ways. In some cases, the best solution may be to protect the entire facility from the voltage sag with a device such as a dynamic voltage restorer. In other cases, it may be more economical to identify particularly susceptible components and protect them alone.
Electrical contactors are often used as a safety device in industrial settings to provide a switch between a machine and the power supply that drives the machine. A typical electrical contactor is a form of a solenoid switch that can be used to connect and disconnect electrical circuits to and from each other. In particular, an electrical contactor is operated by applying an electric potential to an electromagnetic coil, which generates a current flow in the coil that induces a magnetic field to close two or more electrical contacts typically held open via a spring. When the electric potential is removed from the coil, the magnetic field that holds the electrical contacts closed is removed allowing the contacts to spring under tension from the spring. Many production facilities use electrical contactors to control one or more motors. While the motors often have enough inertia to ride-through power line disturbances, contactors may be particularly susceptible to voltage sags. For example, “[o]ne manufacturer has provided data that indicates their line of motor contactors will drop out at 50% voltage if the condition lasts for longer than one cycle. This data should be expected to vary among manufacturers, and some contactors can drop out at 70% normal voltage or even higher.” (McGranaghan et al., “Voltage Sags in Industrial Plants,” IEEE Transactions on Industry Applications, Vol. 29, no. 2, March/April, 1993, quoting M. Sauter, “Voltage fluctuations on Power Systems,” Westinghouse Electric Utility Engineering Reference Book, Distribution Systems, p. 362, 1965).
One approach to addressing the problem of contactor dropout is described in U.S. Pat. No. 5,734,543 to Turner (hereinafter the '543 patent). The '543 patent describes “a mechanism for improving the performance of a solenoid device during a lapse in power quality to the solenoid device from an alternating current (AC) power source. The mechanism comprises a wave shaping device configured to introduce a direct current (DC) component to an alternating current signal provided by the alternating current power source and to output at least part of the alternating current signal modified by the direct current component to the solenoid device.” (The '543 patent at col. 1, line 65 through col. 2, line 6).
Moreover, the mechanism described in the '543 patent is designed to “provide an AC electrical contactor having improved voltage sag ride through capability.” (The '543 patent at col. 1, lines 62-64). Unfortunately, the '543 patent does not address the problem of electrical contactor ride-through in response to a complete dropout of the AC power source (i.e., the AC power source voltage falls to zero for a fraction of a cycle or longer).
Consequently, there is a need for power supply systems and methods that can improve the ride-through capability of an electromagnetic device, such as an electrical contactor, particularly under voltage dropout conditions.